So I use jolt shoulder pads and although comfortable, they're pretty big. I had some old hockey shoulder pads laying around so I cut off the shoulder caps and they're really small and mobile now. Do you think I would be able to use these in a game or practice or whatever or is that just weird?

games: probably not. In the UK and I assume the rest of the world, if it isn't designed for lacrosse it's not legal to wear during games. Even if the kit is pretty similar and is often the same item just rebranded/badged. it's more to do with other players safety than your own, American football pads sometimes are held together with metal buckles which could potentially cause another player a nasty cut for example.

Practice would entirely be at the discretion of the coach(at least in the UK), I wouldn't let the guy with the football pads wear them for training, but if it looks like a lax pad AND POSES NO MORE DANGER TO OTHERS/YOU than lax pads then he probably wouldn't care.